1. Field of the Invention
This invention relates to hydroelectric power-generating devices that use underwater currents to drive electricity-generating rotor assemblies.
2. Description of the Prior Art
The use of underwater power generators for generating electricity from water current flow, such as rivers and oceans, is known in the art. There are two types of prior art ocean devices: stationary turbines and tethered turbines. Stationary turbines are comprised of stationary towers based on the ocean floor. Electricity-generating turbines are mounted on the towers at a fixed depth, with rotor blades facing the flow upstream or downstream of the tower. This type of design has several disadvantages: higher underwater construction costs; the engineering challenges related to installing towers in deep water; reduced current velocity close to the ocean floor resulting in lower power output; and maintenance of underwater systems.
Tethered devices that are anchored to the ocean floor are designed to operate underwater. In some cases, a wing (hydrofoil) provides lift, and/or ballast tanks provide buoyancy in order to keep the devices from descending. Some devices use a buoyancy chamber to regulate their overall buoyancy, thereby adjusting their operating depths in a current stream. Other devices add movable surfaces that serve as an elevator to control the depth of the device. The elevator surface is adjusted to assist the device to dive or ascend, as needed.
By using both local marine current measurements with known global current patterns, a number of sites in the ocean have been identified for deployment of marine current generating devices, representing several thousand gigawatts of potential generation. Many countries throughout the world rely heavily on importing fuel for generating electricity, and they lack other viable renewable energy sources. In view of current population growth (increasing by 1.5 million humans per week), a perilous trend in climate change, growing demand for natural gas and petroleum, and the increasing difficulty in finding and developing new petroleum fields, creates an urgency for developing and deploying new sustainable and cost-effective technologies to transition energy resources and consumption away from carbon-based fossil fuels.
Most marine current generation technologies are migrating to the use of submerged systems. Energy can be extracted from the ocean using submerged turbines that are similar in function to wind turbines, converting energy through the process of hydrodynamic rather than aerodynamic lift or drag. These turbines have rotor blades, generators for converting rotational power into electricity, and means for transmitting the electrical current generated to the shore-based electrical grid.
Today, both horizontal and vertical axis turbines are generally considered for producing power from ocean currents. Ocean current power systems are at an early stage of development; only a few prototypes of small scale and a few demonstration units having been tested or shown to date and most devices are below a 2 MW generating capacity rating.
Numerous patents have been issued related to systems for producing energy from ocean currents. Some of the patents describe devices using active stability, depth and rotor control, which increases cost and reduces reliability.
Prior art designs describe complex active systems such as control surfaces, variable ballast, variable pitch, winching systems, or mechanical means for raising/lowering the structure. In a moored system with harsh environmental and structural loading from ocean tidal currents, gyres (steady ocean currents), and eddies, failures from these control methods could result in the inability to access the device, or, in the worst cases, complete loss of the structure, or hazard to navigation for vessels using the area. In addition to the inherent risks, these controls lack simplicity; they provide many variables and opportunities for failure, along with the additional costs associated with these types of control methods. Lower reliability, lower operating availability for power production, and higher maintenance requirements mean that the designs shown in the prior art are not cost-effective.
To satisfy customer needs, hydrokinetic devices must provide a low cost of energy, high reliability and high operating availability to the grid, and they should have a service life greater than 20 years and a high safety factor in operation and maintenance. Maintenance costs for offshore structures require a significantly different mindset than that for onshore power plants. For example, accessing the mechanical components of submerged systems can require specialized crews, divers, autonomous underwater vehicles (AUV's), Remotely Operated Vehicles (ROV's), vessel costs for mobilization and demobilization, and related fuel costs. To be comparable to other power-generating technologies on a cost-of-energy basis, the scheduled visits to perform maintenance should be on the order of five years. Servicing should be on the ocean surface to minimize risks to the operating crews. Sub-systems must, therefore, be simple and reliable, and they should use proven components to achieve low cost-of-energy targets.
It is therefore desirable to take a systematic approach to provide a simple and reliable system, one that is easy to maintain and service, with low cost-of-energy (COE) and a service life longer than 20 years.
What is needed is a simplified, tethered, underwater, current-driven rotor assembly coupled to a power-generating system, which, in a cost-effective manner, passively adjusts to varying ocean currents to limit structural loads without active control surfaces, with minimal effect on stability, safety, and power-generating performance.
In order to reduce installation and maintenance costs, use of underwater structures and moving parts should be minimized. A method of safely and economically mooring and installing the underwater device in its operational position should be provided, along with a procedure to safely bring the device to the surface for maintenance or for replacement of components. Variable blade pitch should be eliminated to reduce the potential of pitch system failure and other maintenance issues. Complete emergency shutdown of the device, including stopping rotor blade rotation, should be possible, and the device should have fail safe depth control which prevents unplanned surfacing.